Photocontrolled Interconversion of Cationic and Radical

Jul 26, 2017 - The ability to combine two polymerization mechanisms in a one-pot setup and switch the monomer selectivity via an external stimulus pro...
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Photocontrolled Interconversion of Cationic and Radical Polymerizations Veronika Kottisch, Quentin Michaudel, and Brett P. Fors* Cornell University, Ithaca, New York 14853, United States S Supporting Information *

polymerization process.5a Notably, using a reducing photocatalyst, Boyer4a−h and others4i,j have shown that similar trithiocarbonates can be used for the photocontrolled radical polymerization of acrylates. We hypothesized that a combination of these photocontrolled cationic and radical processes would enable switching of the monomer selectivity in situ with light. Specifically, excitation of an oxidizing photocatalyst would generate a propagating cation and allow the selective polymerization of vinyl ethers. Conversely, excitation of a reducing photocatalyst would induce a radical mechanism selective for the polymerization of acrylates (Scheme 1). This

ABSTRACT: The ability to combine two polymerization mechanisms in a one-pot setup and switch the monomer selectivity via an external stimulus provides an excellent opportunity to control polymer sequence and structure. We report a strategy that enables monomer incorporation to be determined via the selection of the wavelength of light through selective activation of either cationic or radical processes. This method enables the synthesis of varying polymeric structures under identical solution conditions but with simple modulation of the external stimulus. Additionally, changes in the ratios of the two photocatalysts afford complementary chemical control over these reactions to design elaborated polymeric structures. Our strategy takes advantage of the unique regulation that can be accessed through light.

Scheme 1. Switching the Polymerization Mechanism and Monomer Selectivity by Changing the Wavelength of Light Irradiation

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uring the past two decades, advances in polymer chemistry have enabled the synthesis of macromolecules with well-defined structures. However, an opportunity remains to develop strategies that offer precise control over polymer structure in a single efficient process. Specifically, the ability to switch the polymerization mechanism and hence the monomer selectivity in situ by using an external stimulus is a grand challenge. Achieving such a breakthrough would streamline the synthesis of functional materials. Light is one of the most powerful external stimuli and may be the key to addressing this challenge.1 A renaissance of photochemistry has recently taken place in materials science, and a wide range of polymerization techniques that enable precise control of polymer chain growth with light have been developed.2 The majority of these processes have been applied to radical polymerizations,3,4 but recent advances have extended light regulation to cationic polymerizations.5 Using these photopolymerizations, Boyer6 as well as Goto and Kaji7 have promoted two in situ polymerization processes with various wavelengths of light using a bifunctional initiator to form diblock copolymers. However, these methods do not allow conversion between polymerization mechanisms or monomer selectivity at a single propagating chain end. The ability to change the monomer selectivity in situ with light would provide a major opportunity to control polymer structure. Our group recently developed a photocontrolled cationic polymerization of vinyl ethers. The oxidation of a trithiocarbonate chaintransfer agent (CTA) with a photocatalyst yielded a cation that could promote the controlled polymerization of vinyl ethers in a reversible addition−fragmentation chain transfer (RAFT) © 2017 American Chemical Society

mechanism would enable monomer incorporation to be determined via the selection of the wavelength of light and give precise control of the polymer structure through the use of an external stimulus. Beyond photocontrolled polymerizations, Satoh and Kamigaito8 independently developed systems in which both radical and cationic polymerizations are concurrently active to allow copolymerizations of vinyl ethers and acrylates. Under these conditions, the trithiocarbonate chain end efficiently interconverts between cationic and radical mechanisms to yield multiblock structures. The data obtained with this system provide support for our hypothesis that we could efficiently Received: June 27, 2017 Published: July 26, 2017 10665

DOI: 10.1021/jacs.7b06661 J. Am. Chem. Soc. 2017, 139, 10665−10668

Communication

Journal of the American Chemical Society

reducing catalyst. Complex 3 has been used previously in photoinduced electron transfer RAFT (PET-RAFT)4a and photocontrolled atom transfer radical polymerization (ATRP)3a processes. Irradiation with blue light produced poly(methyl acrylate) with a number-average molar mass (Mn) of 9.7 kg/ mol and a dispersity (Đ) of 1.26 within 23 h (Figure 1b). Notably, this result illustrates that radical polymerization can be efficiently initiated from the poly(IBVE) chain ends grown under our cationic polymerization conditions on the basis of their structural similarity with trithiocarbonate 1, which is imperative for switching between cationic and radical polymerizations. The radical polymerization with both IBVE and MA was probed using conditions analogous to those in the experiment in Figure 1a but substituting 2 with the reducing photocatalyst 3. Exposure to blue light yielded a 9.3 kg/mol poly(IBVE-rMA) random copolymer with ∼30% incorporation of IBVE (Figure 1c). 13C and 1H NMR analyses showed that all of the IBVE units within the copolymer chain were flanked by two MA monomers. This result is consistent with other radical copolymerizations of these two monomers.8 With an understanding of the cationic and radical photopolymerizations with both IBVE and MA, we next investigated the use of light to switch between polymerization mechanisms. For these experiments, we used a combination of photocatalysts 2 and 3 with an equimolar mixture of IBVE and MA.9 On the basis of the absorption windows of the two catalysts, we hypothesized that we could selectively excite the oxidizing photocatalyst 2 in the presence of 3 with 520 nm light, which would result in the exclusive cationic polymerization of IBVE (Figure 2).4a In support of this hypothesis, the use of green

switch the polymer chain growth from a cationic to a radical mechanism using the photocontrolled polymerizations discussed above. Our initial studies investigated the monomer selectivity under our cationic polymerization conditions. Using trithiocarbonate 1 as the CTA and 2,4,6-tris(p-methoxyphenyl)pyrylium tetrafluoroborate (2) as the oxidizing photocatalyst, we irradiated an equimolar reaction mixture of isobutyl vinyl ether (IBVE) and methyl acrylate (MA) with blue lightemitting diodes (LEDs). After 2 h, a well-controlled poly(IBVE) homopolymer was formed, and no polymerization of MA was observed (Figure 1a). Notably, negligible conversion

Figure 2. UV−vis absorption spectra of 2 and 3.

LEDs yielded a well-controlled IBVE homopolymer with no evident MA conversion even after irradiation for several hours (Figure 3a(i)).10 Under the same conditions, the light source was then switched to blue LEDs after almost 80% consumption of IBVE, which led to the excitation of 3 and initiated the radical polymerization of MA to give a poly(IBVE-b-MA) tapered diblock copolymer (Figure 3a(ii) and Figure 3b). This result shows that the polymerization mechanism can indeed be switched in situ from cationic to radical by changing the wavelength of the light.11 Notably, using the same reaction conditions with only bluelight irradiation yields a multiblock copolymer with Mn = 11.7 kg/mol and Đ = 1.43 (Figure 3a(iii) and Figure 3c). Because both photocatalysts are excited by 450 nm light, the radical and cationic polymerizations are concurrently active under such conditions, which causes the polymer chain end to switch between the two polymerization types (Figure 2). Data obtained by monitoring the conversion over the course of the

Figure 1. (a) Homopolymerization of IBVE in the presence of MA under standard oxidizing conditions. (b) Radical polymerization of MA in the presence of CTA 1 and Ir(ppy)3 (3). (c) Statistical copolymerization of IBVE and MA under blue light during radical copolymerization.

of MA was detected even after several days under these conditions. These results both clearly demonstrate that our photocontrolled cationic polymerization conditions using catalyst 2 lead to the selective polymerization of IBVE in the presence of MA and, critically, establish that the putative radical species5a formed under these conditions do not initiate the polymerization of MA. Further studies probed photocontrolled radical polymerizations of MA with 1 as the CTA and Ir(ppy)3 (3) as the 10666

DOI: 10.1021/jacs.7b06661 J. Am. Chem. Soc. 2017, 139, 10665−10668

Communication

Journal of the American Chemical Society

Figure 4. (a) Reaction conditions for diblock copolymer synthesis under blue light. (b) Conversion and GPC trace of in situ tapered diblock copolymer formation. (c) Conversion and GPC trace of in situ nontapered diblock copolymer formation.

Figure 3. (a) Exposing an equimolar mixture of MA and IBVE (i) to green light selectively induces cationic polymerization, whereas (ii) exposure to green and then blue light creates a tapered diblock copolymer and (iii) exposure to blue light yields a multiblock copolymer. (b) Gel permeation chromatography (GPC) traces and monomer conversion of the diblock copolymer. (c) GPC traces and monomer conversion of in situ multiblock copolymer formation.

consumption of IBVE reached 100% before MA polymerization started (Figure 4c). In conclusion, we have successfully coupled photocontrolled cationic and radical polymerization processes to enable switching of the polymerization mechanism and monomer selectivity by changing the wavelength of light. Under identical solution conditions, we could discriminatorily synthesize a homo-, diblock, or multiblock polymer by adjusting our external stimulus, light. The ability to use light as a tool to manipulate the identity of propagating chain ends can simplify a multistep synthesis to a one-pot procedure and, importantly, enable the synthesis of novel polymeric materials. Notably, facile switching from cationic to radical polymerization with light still remains a difficult challenge in this system. However, we expect this study to lay the groundwork for a new strategy to control polymer structure and architecture with light.

reaction highlight the copolymerization of the two monomers (Figure 3c). Crossover peaks from the poly(IBVE) block to a poly(methyl acrylate) block and vice versa were clearly observed in the 13C NMR spectrum and showed that these conditions led to a pentablock copolymer.8b Similar experiments were performed with chloroethyl vinyl ether and a variety of acrylates with comparable levels of control (see the Supporting Information). The three experiments in Figure 3 illustrate that we can tune the polymer structure with light using photocontrolled polymerizations. These experiments were performed under identical conditions in solution and varied only in the exposure to external light stimuli. With this variation, we can selectively induce one polymerization mechanism over the other with light and accordingly manipulate the final polymer structure. This new level of control brings us another step closer to lightmediated sequence control of polymers.12 Along with light, the catalyst ratio can also be varied to control the polymer structure. Using double the amount of 2 with respect to 3 changed the polymer from a multiblock copolymer, as seen in Figure 3c, to a tapered diblock copolymer (Figure 4a(i)). Monitoring the conversion of both monomers over time revealed that the polymerization of IBVE started immediately upon irradiation, whereas the polymerization of MA had a large induction period and commenced only after the conversion of IBVE had reached 80% (Figure 4b). By further increasing the amount of 2, we created a nontapered diblock copolymer (Figure 4a(ii)). Under these conditions, the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b06661. General experimental considerations, experimental procedures, and additional supporting data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Brett P. Fors: 0000-0002-2222-3825 Notes

The authors declare no competing financial interest. 10667

DOI: 10.1021/jacs.7b06661 J. Am. Chem. Soc. 2017, 139, 10665−10668

Communication

Journal of the American Chemical Society

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(9) In this system, electron transfer between the two catalysts could be occurring; future studies will investigate the mechanism and possible influence of this electron transfer on these polymerizations. (10) During irradiation with green LEDs it is important to exclude all other light sources to avoid any radical polymerization. (11) Switching from radical to cationic polymerization by changing the wavelength of light remains difficult because formation of the cation at the acrylate chain end is unfavorable. (12) Lutz, J.-F.; Ouchi, M.; Liu, D. R.; Sawamoto, M. Science 2013, 341, 1238149.

ACKNOWLEDGMENTS This work was supported by Cornell University, made use of the NMR Facility at Cornell University, and was supported in part by the NSF under Award CHE-1531632. B.P.F. thanks 3M for a Nontenured Faculty Award. We thank Dr. I. Kerestzes for help with NMR spectroscopy.

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DOI: 10.1021/jacs.7b06661 J. Am. Chem. Soc. 2017, 139, 10665−10668